Downloaded from www.microbiologyresearch.org by IP: 54.70.40.11 On: Sat, 08 Dec 2018 04:23:23 The X proteins of bornaviruses interfere with type I interferon signalling Jonas Johansson Wensman, 1,2 Muhammad Munir, 1 Srinivas Thaduri, 1 Katarina Ho ¨ rnaeus, 1 3 Muhammad Rizwan, 1 Anne-Lie Blomstro ¨m, 1 Thomas Briese, 3 W. Ian Lipkin 3 and Mikael Berg 1 Correspondence Jonas Johansson Wensman Jonas.Wensman@slu.se Received 14 August 2012 Accepted 22 October 2012 1 Swedish University of Agricultural Sciences, Department of Biomedical Sciences and Veterinary Public Health, Section of Virology, PO Box 7028, SE-750 07 Uppsala, Sweden 2 Swedish University of Agricultural Sciences, Department of Clinical Sciences, Division of Ruminant Medicine and Veterinary Epidemiology, PO Box 7054, SE-750 07 Uppsala, Sweden 3 Columbia University, Mailman School of Public Health, Center for Infection and Immunity, 722 West 168th Street, 10032 New York, NY, USA Borna disease virus (BDV) is a neurotropic, negative-stranded RNA virus causing persistent infection and progressive neurological disorders in a wide range of warm-blooded animals. The role of the small non-structural X protein in viral pathogenesis is not completely understood. Here we investigated whether the X protein of BDV and avian bornavirus (ABV) interferes with the type I interferon (IFN) system, similar to other non-structural proteins of negative-stranded RNA viruses. In luciferase reporter assays, we found that the X protein of various bornaviruses interfered with the type I IFN system at all checkpoints investigated, in contrast to previously reported findings, resulting in reduced type I IFN secretion. INTRODUCTION Viruses comprise the largest group of biological entities on earth (Koonin et al., 2006). During evolution, host cells have evolved defence mechanisms against viral infections and viruses have, consequently, developed effective meth- ods to escape host defence mechanisms (Garcı´a-Sastre & Biron, 2006). First in line of the host defence mechanisms is the type I interferon (IFN)-mediated innate immune response, commonly activated upon RNA virus infection. IFNs are secreted cytokines that are able to stimulate expression of hundreds of different genes and may thus affect several steps of the viral life cycle (Sen, 2001; Stark et al., 1998). The IFN-mediated antiviral programme is initiated by viral activation of cellular sensors. Toll-like receptors (TLRs), associated chiefly with endosomes, recognize ssRNA and dsRNA, as well as viral DNA. Retinoid-inducible gene I (RIG-I), protein kinase R (PKR) and melanoma differenti- ation-associated gene 5 (MDA5) are cytoplasmic sensors that recognize dsRNA (Baum & Garcı´a-Sastre, 2010; Garcı ´a-Sastre & Biron, 2006; Sen, 2001). The activation of viral sensors triggers downstream signalling molecules, culminating in activation of constitutively expressed transcription factors, such as activator protein 1 (AP-1), nuclear factor kappa B (NF-kB) and IFN regulatory factors 3 and 7 (IRF3/7). NF-kB and AP-1 strongly upregulate type I IFN expression (Haller et al., 2006). The secreted type I IFNs sequentially activate the IFN-stimulated response element (ISRE) in an autocrine and/or paracrine manner. The activated ISRE leads to the transcription of hundreds of IFN-stimulated genes (ISGs) that collectively determine the antiviral state of the host cell. Borna disease virus (BDV) is the causative agent of Borna disease, a fatal neurological disease mainly affecting horses and sheep (Ludwig & Bode, 2000). The virus is a non- segmented, negative-stranded RNA virus that belongs to the order Mononegavirales (Lipkin & Briese, 2006). Due to its nuclear site of replication (Briese et al., 1992) and use of the cellular RNA splicing machinery for gene expression (Cubitt et al., 1994; Schneider et al., 1994), BDV is the solitary member of the family Bornaviridae. BDV was first discovered in horses and sheep, although natural infection has been reported in a wide range of animals, including cats (Wensman et al., 2008). Most BDV strains have high genetic similarity (95–99 %), where the highest genetic divergence of around 15 % was found for a strain (No/98) that was isolated from an Austrian horse (Nowotny et al., 2000). Recently, even more divergent viruses have been described in psittacine birds with proventricular dilatation 3Present address: Swedish University of Agricultural Sciences, Department of Animal Breeding and Genetics, Animal Genetics Laboratory, PO Box 7023, SE-750 07 Uppsala, Sweden. The GenBank/EMBL/DDBJ accession number for the X gene sequence of feline BDV is JX477137. Journal of General Virology (2013), 94, 263–269 DOI 10.1099/vir.0.047175-0 047175 G 2013 SGM Printed in Great Britain 263